U.S. patent application number 11/916877 was filed with the patent office on 2009-11-12 for titanium, titanium alloy and niti foams with high ductility.
Invention is credited to Jozef Cooymans, Jan Luyten, Steven Mullens, Ivo Thijs.
Application Number | 20090280022 11/916877 |
Document ID | / |
Family ID | 35134053 |
Filed Date | 2009-11-12 |
United States Patent
Application |
20090280022 |
Kind Code |
A1 |
Mullens; Steven ; et
al. |
November 12, 2009 |
TITANIUM, TITANIUM ALLOY AND NiTi FOAMS WITH HIGH DUCTILITY
Abstract
A method for manufacturing a high ductility Ti-, Ti-alloy or
NiTi-foam, meaning a compression strain higher than 10%, includes:
preparing a powder suspension of a Ti-, NiTi- or Ti-alloy powder,
bringing the said powder suspension into a desired form by
gelcasting to form a green artefact. The method also includes a
calcination step wherein the green artefact is calcined, and
sintering the artifact. The calcination step includes a slow
heating step wherein said green artefact is heated at a rate lower
or equal to 20.degree. C./hour to a temperature between 400.degree.
C. and 600.degree. C. and the Ti-, NiTi- or Ti-alloy powder has a
particle size less than 100 .mu.m. A high ductility Ti-, Ti-alloy
or NiTi foam, with a compression higher than 10%, with a
theoretical density less than 30%, pore size (cell size) between 50
to 1000 .mu.m can be obtained with such a method.
Inventors: |
Mullens; Steven; (Zichem,
BE) ; Thijs; Ivo; (Mol, BE) ; Cooymans;
Jozef; (Mol, BE) ; Luyten; Jan; (Molenstede,
BE) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
35134053 |
Appl. No.: |
11/916877 |
Filed: |
June 7, 2006 |
PCT Filed: |
June 7, 2006 |
PCT NO: |
PCT/BE06/00066 |
371 Date: |
July 1, 2008 |
Current U.S.
Class: |
419/2 ;
75/228 |
Current CPC
Class: |
B22F 3/1017 20130101;
A61F 2/28 20130101; C22C 1/08 20130101; B22F 2998/00 20130101; C22C
14/00 20130101; B22F 3/1021 20130101; B22F 3/1112 20130101; C22C
19/03 20130101; B22F 3/1137 20130101; Y10T 428/12479 20150115; B22F
2998/00 20130101; B22F 3/1121 20130101 |
Class at
Publication: |
419/2 ;
75/228 |
International
Class: |
B22F 3/11 20060101
B22F003/11 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 7, 2005 |
EP |
05447131.3 |
Claims
1. A method for manufacturing a high ductility Ti-, Ti-alloy or
NiTi-foam, wherein the metallic foam can be deformed more than 10%
in compression without rupture, comprising: preparing a powder
suspension of a Ti-, NiTi- or Ti-alloy powder, bring said powder
suspension into a desired form by gelcasting to obtain a green
artefact, a calcination step wherein said green artefact is
calcined, and a sintering step wherein said artefact is sintered
wherein calcination step comprises a slow heating step wherein said
green artefact is heated at a rate lower or equal to 20.degree.
C./hour to a temperature between 400.degree. C. and 600.degree. C.
and wherein the Ti-, NiTi- or Ti-alloy powder has a particle size
less than 100 .mu.m.
2. The method according to claim 1, wherein said calcination step
is performed under an inert atmosphere.
3. The method according to claim 1, wherein said calcination step
is performed at a pressure less than 10.sup.-3 mbar.
4. The method according to claim 1, further comprising a
presintering step between the calcination step and the sintering
step, comprising heating up the artefact until a temperature of
between 900 and 1000.degree. C. is reached.
5. The method according to claim 1, wherein the sintering step
comprises slow heating of the artefact to a temperature between
1200.degree. C. and 1500.degree. C., and keeping said artefact at
said temperature for a predetermined period of time.
6. The method according to claim 1, wherein the powder comprises
less than 0.03 weight % C, less than 0.8 weight % O, less than 0.5
weight % N, less than 0.1 weight % Fe and less than 0.1 weight %
Si.
7. The method according to claim 1, wherein the powder suspension
is brought into the desired form using gelcasting, resulting in
Ti-, Ti-alloy or NiTi-foams with porosities between 60 to 95% and
with pore sizes between 50 to 1000 .mu.m.
8. A high ductility Ti-, Ti-alloy or NiTifoam with a theoretical
density less than 30% and pore size between 50 to 1000 .mu.m.
Description
FIELD OF THE INVENTION
[0001] The present invention is related to high ductility titanium,
titanium alloy or NiTi foams, in particular for use in biomedical
or composite structures. The invention relates also to the use of
such foams as bone replacement or healing aid.
STATE OF THE ART
[0002] Titanium, titanium alloys and NiTi are commonly used for
their high strength/weight ratio, their high resistance to
corrosion and relatively low elasticity modulus, and their
biocompatibility. The mechanical properties of Ti-foams are very
close to bone properties and therefore, it seems the select
material for bone replacement or healing aid. However, this
application requires high ductility foams. Ductility is more
commonly defined as the ability of a material to deform easily upon
the application of a tensile force, or as the ability of a material
to withstand plastic deformation without rupture, breaking or
fracturing. Ductility may also be thought of in terms of bend
ability and crushability. Ductile materials show large deformation
before fracture. The lack of ductility is often termed
brittleness.
[0003] Titanium is very reactive with a lot of compounds if heated
above 300.degree. C. Due to interstitial incorporation of elements
like N, O, C or H in the Ti, a brittle foam is obtained, lacking
strength and ductility (or insufficient fatigue properties) to
serve as a high-quality bone replacement.
[0004] Some manufacturing procedures of these metallic foams
require large relative quantities of organic materials (which are
used as additives in the suspension preparation or which serve as
template for the porous structure like e.g. in the polyurethane
replica technique). As such, the residual contamination of elements
like N, O, C or H will be much higher, resulting in an
embrittlement of the metallic foam. Typical ductility values for Ti
foams manufactured by the polyurethane replica technique is about
3% in strain.
[0005] On the other hand, low contamination with N, O, C and thus
high ductility can be seen as a consequence of the use of the
gelcasting procedure as the shaping method.
AIMS OF THE INVENTION
[0006] The present invention aims to provide a method for the
production of high-ductility Ti, Ti-alloy and NiTi foams.
[0007] The present invention also aims at providing a
high-ductility, highly porous (>70% total porosity) Ti, Ti-alloy
and NiTi foams. Preferably, a minimum plastic deformation of 10%
without rupture, breaking or fracturing should be achieved.
SUMMARY OF THE INVENTION
[0008] The present invention concerns a method for manufacturing a
high ductility Ti-, Ti-alloy- or NiTi-foam, meaning that the
metallic structure can be deformed more than 10% in compression
without rupture, comprising the steps of [0009] Preparing a powder
suspension of a Ti-, NiTi- or Ti-alloy powder, [0010] Bring said
powder suspension into a desired form by gelcasting to form a green
artefact, [0011] A calcination step wherein said green artefact is
calcined, and [0012] Sintering said artefact, characterised in that
said calcination step comprises a slow heating step wherein said
green artefact is heated at a rate lower or equal to 20.degree.
C./hour to a temperature between 400.degree. C. and 600.degree. C.
and that the Ti-, NiTi- or Ti-alloy powder has a particle size less
than 100 .mu.m.
[0013] Preferably, said calcinations step is performed under an
inert atmosphere or at a pressure less than 10.sup.-3 mbar.
[0014] The method according to the present invention can further
comprise a presintering step comprising heating the artefact until
a temperature of between 900 and 1000.degree. C. is reached. It can
also comprise a sintering step comprising slow heating of the
artefact to a temperature between 1200.degree. C. and 1500.degree.
C., and keeping said artefact at said temperature for a
predetermined period of time.
[0015] In a preferred embodiment, the powder comprises less than
0.03 weight % C, less than 0.8 weight % O, less than 0.5 weight %
N, less than 0.1 weight % Fe and less than 0.1 weight % Si.
[0016] The powder suspension can be brought into the desired form
using gelcasting.
[0017] Another aspect of the present invention concerns the high
ductility of the highly porous Ti-, Ti-alloy or NiTi-foams with a
theoretical density less than 30%.
[0018] High ductility means that these Ti-foams can be deformed in
compression for more then 10%. A certain plasticity is a minimum
requirement for load bearing implants.
SHORT DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 represents a flowchart of the general method
according to the present invention.
[0020] FIG. 2 represents the effects of sandblasting on the surface
of a Ti-foam which was manufactured according to the present
invention. The artefact surface is shown before (FIG. 2 a) and
after (FIG. 2 b) the sandblasting operation.
[0021] FIG. 3 shows a typical pore size distribution of a Ti foam
structure obtained by gelcasting according to the present
invention.
[0022] FIG. 4 shows the relation between stress and deformation for
several sintering temperatures for a foam according to the present
invention and compares it with these of a Ti foam manufactured by
PU replica technique.
[0023] FIG. 5 shows the shape memory effect of a NiTi foam
structure obtained according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] A more detailed description of the present invention is
presented by means of examples and figures. The general process
according to the present invention is given in FIG. 1.
[0025] The powder suspension is prepared by mixing water and
Ti-powder (or powder of a Ti-alloy such as Ti6Al4V, or
NiTi-powder), with a powder volume between 20 and 50 vol %) with
additives such as dispersing agents and gelling aids. The skilled
person is well acquainted with these additives and can easily
prepare a suitable powder suspension according to this first step
of the process, suitable for the gelcasting forming process. Also,
the viscosity of the suspension can be easily adapted to the
desired range suitable for gelcasting. Examples of gelling aids and
other additives, which can be used for obtaining a stable Ti-,
Ti-alloy or NiTi-suspension according to the known methods as
presented in the forming step of FIG. 1, are below:
Gelling aids: [0026] Gelatine [0027] Agarose [0028] Other
hydrocolloids or biobinders (pectins, starch, xanthane, carragene,
. . . ) Other additives [0029] Cellulose-derived (e.g.
hydroxypropyl methylcellulose) [0030] Alginate and its derivatives
[0031] Dispersion agents (such as Targon.TM. 1128, DarvanC.TM., . .
. ) [0032] Foaming agents
[0033] In order to obtain a suitable suspension for the process
according to the present invention, the particle size of the
metallic powder should be less than 100 .mu.m, the particle shape
should be spherical or irregular (depending on the milling method)
and of high purity, with less than 0.03 weight % C, less than 0.8
weight % 0, less than 0.5 weight % N, less than 0.1 weight % H,
less than 0.1 weight % Fe and less than 0.1 weight % Si. The
particle size of the Ti-powder has to be between 10 and 100 .mu.m
for the following reasons:
[0034] Ti-powder with particle size under 10 .mu.m has a too high
oxygen quantity (TiO.sub.2 on their surface); On the other hand
this kind of Ti-powders are too pyrophore for easy handling.
Ti-powder, particle sizes above 100 .mu.m, gives problems in the
preparation of the slurry as sedimentation of the suspension
occurs.
[0035] Gelcasting with hydrocolloids or biobinders as gelling agent
produces a highly porous structure (70-90% of the theoretical
density), an open pore network in which the pore dimensions ranges
between 50 and 1000 .mu.m (depending on the powder suspension and
process parameters such as type and quantity of foaming agent,
viscosity of the suspension, foaming procedure, . . . ) and a high
interconnectivity.
[0036] The calcination step is preferably executed in two steps. In
the first step, a slow heating (maximum 20.degree. C. per hour)
until a temperature of 400 to 600.degree. C. is obtained
(preferably less than 500.degree. C.). A too high calcination rate
has to be prevented:
[0037] it prevents cracking of the green product
[0038] carbon residues have to be minimized because carbon take-up
by the Ti-scaffold is directly related to his brittleness
[0039] Advantageously, this is done under inert atmosphere such as
argon or at low pressure (minimum of 10.sup.-3 mbar). During this
first step, most of the organic material present will be burnt
away.
[0040] The second step of the calcination step is a presintering
step until a temperature of 900 to 1000.degree. C. is reached. This
results in a structure where the Ti powder particles already start
to sinter together, allowing handling of the structure. Preferably,
this step is performed under a vacuum of at least 10.sup.-4 mbar on
a Mo plate or on a Y.sub.2O.sub.3-(coated) substrate.
[0041] Next, the obtained structure is sintered by placing it on Mo
or a Y.sub.2O.sub.3-(coated) strips or rolls, allowing a further
shrinking of the structure. Again, a high vacuum of more than
10.sup.-4 mbar is used. If necessary, the Ti foam can be
co-sintered to a dense Ti structure.
[0042] Optionally, a working step can be used in case it is
necessary to remove e.g. reaction zones situated at the contact
points with the Mo support material or when other structure
corrections (e.g. sawing) are desired.
[0043] Also, a sandblasting operation can be useful to further
remove the semi-closed pore windows at the surface of the artefact.
The result of such a sandblasting operation can be seen in FIG.
2.
EXAMPLES
1. Ti Foam Structure Obtained by Gelcasting
[0044] A suspension is prepared using 300 g Ti (Cerac; 99.5% pure;
-325 mesh), 201 g H.sub.2O, 6.4 g Agar (3.18% on H.sub.2O), 6 g
Tergitol TMN10 (2% on Ti), 3 g Triton (1% on Ti) and 0.36 g
Ammonium Alginate (0.18% on H.sub.2O). It is mixed during 6 minutes
at 70.degree. C. to obtain a fluid foam. The foam is cast into a
mould and cooled down until the structure is gelled. After
demoulding, the structure is dried at atmospheric pressure and room
temperature.
[0045] The structure is then calcined (10.sup.-3 mbar, 20.degree.
C./h until 500.degree. C., 2 hours isotherm, in the presence of Zr
metal) and sintered (10.sup.-4 mbar, 5.degree. C./min to
1200-1500.degree. C.; 2 h isotherm). An open-structured Ti foam
with pore size between 50 and 1000 .mu.m is obtained. A typical
pore size distribution is shown in FIG. 3. The influence of the
sinter temperature on the stress-strain curve of gelcasted Ti-foams
is presented in FIG. 4. As a comparison, the stress-strain curves
for ti-foams manufactured by the polyurethane technique is
presented, showing the great difference in ductility between the 2
techniques. The tests were performed on Ti-foams, cutted in
cylinders of diameter 16 mm and a height of 20 mm, compressed on a
conventional Instron Machine, with a compression speed of 2
mm/min.
[0046] Properties of the obtainable structures include (depending
on sintering temperature):
TABLE-US-00001 E-modulus (GPa) 1500.degree. C. 2.5 1425.degree. C.
2.1 1350.degree. C. 1.8 1250.degree. C. 1.5 Specific surface
(m.sup.2/g) 1500.degree. C. 0.7 1250.degree. C. 1.3 Porosity (% TD)
1500.degree. C. 70% 1250.degree. C. 75%
as can be seen from the stress-strain curves, deformation higher
than 10% is obtained.
2. TiAl64V Foams
[0047] A Ti-6Al-4V-suspension was prepared, similar to the
composition in example 1, using a Ti-6Al-4V powder with a similar
particle size distribution. After gelcasting and sintering,
cylindric samples were cut and compressed (condition similar to
example 1: sample dimensions: diameter 16 mm and height 20 mm;
tests were performed on a Instron-equipment using a compression
speed of 2 mm/min). For a foam density of 17% of the theoretical
density, the compression stress was 34 MPa and a strain of more the
24% could be obtained.
3. NiTi Foams
[0048] NiTi can also be used for producing foams. An example is a
NiTi-foam created with the gelcasting technique.
[0049] Differential Scanning Calorimetry (DSC, TA Instruments 2920)
was used to measure the shape memory effect of such a foam. The
transition temperature is clear from FIG. 5. These materials are
very promising for biomedical applications as the transitions occur
at body temperature.
[0050] The existence of this Shape Memory Effect (SME) and the
sharp transition peak in the DSC-profile which could be
demonstrated is an indirect proof of a low contamination of O, N,
C.
[0051] Such low contamination as specific property of the
gelcasting process, will result in a high ductility, even further
improved by the pseudo-elasticity of the alloy used.
* * * * *